There have been various proposals to create wireless networks to provide Broadband Wireless Access (BWA). These networks can offer an alternative to conventional wired networks which are based on cable or Digital Subscriber Line (DSL) technologies, and can also be used to provide broadband access to areas where wired networks do not exist. Worldwide Interoperability for Microwave Access (WiMAX), set out in IEEE 802.16-2004, specifies a Wireless MAN Air Interface for ‘fixed’ wireless metropolitan area networks, i.e. networks with static terminals.
A development of IEEE 802.16 is IEEE 802.16e (Mobile WiMAX, now adopted as IEEE 802.16-2005) which provides a common wide area broadband radio access technology for broadband networks which may include static and mobile terminals. The Mobile WiMAX Air Interface uses Orthogonal Frequency Division Multiple Access (OFDMA) for improved multi-path performance in non-line-of-sight environments. An overview can be found in the white paper document “Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation”, Feb. 21, 2006, prepared on behalf of the WiMAX Forum and available at http://wimaxforum.org.
It has been realised that situations arise where terminals cannot be adequately served by a direct path to a base station. Therefore, a further developement of IEEE 802.16 is to support multiple-hop paths between a base station and a fixed or mobile terminal. This is known as IEEE 802.16j, or Mobile Multihop Relay (MMR). FIG. 1 shows an example of a wireless network 10 which uses multi-hop transmission paths. A base station BS serves terminals MS1, MS2, MS3. Terminal MS1 is served directly by the base station BS via a single hop transmission path 2. Terminal MS2 is served by a two-hop transmission path 3, 4 via a relay station RS1. Terminal MS3 is served by a three-hop transmission path 5, 6, 7 via relay stations RS2 and RS3. A multi-hop transmission path may be needed when a single hop transmission path does not offer a sufficient quality. This can be due, for example, to a significant physical obstruction in the line-of-sight path between a base station BS and a terminal, such as the hill 8 shown between base station BS and terminal MS2. Relay stations can also be positioned at the edge of the normal coverage area of a base station to extend the coverage area of the base station. Base stations BS are interconnected via wired, or wireless, backhaul links 11 to a core network 12, which interconnects with other networks 14, such as data networks, the Internet or the PSTN.
FIG. 2 shows the structure of an overall time-division duplex (TDD) frame defined by IEEE 802.16e (IEEE 802.16-2005). Base stations within the system alternately transmit to terminals on a downlink and receive from terminals on an uplink. Each frame is divided into a downlink part (DL) and an uplink part (UL). Time is shown along the horizontal axis and frequency is shown along the vertical axis. IEEE 802.16e uses an OFDM modulation scheme, with a set (e.g. 1024) of OFDM sub-carriers. Consequently, the horizontal time axis corresponds to OFDM symbols and the vertical frequency axis corresponds to OFDM sub-carriers. As the system serves multiple terminals, this scheme is a form of Orthogonal Frequency Division Multiple Access (OFDMA). The downlink part begins with a ‘preamble set’ 31. This comprises a preamble, a Frame Control Head (FCH), a downlink map (DL-MAP) and an uplink map (UL-MAP). The preamble is used for synchronisation, and is the first OFDM symbol of the frame, extending across all OFDM sub-channels. The Frame Control Head (FCH) follows the preamble and provides the frame configuration information such as MAP message length and coding scheme and usable sub-channels. The DL-MAP carries control information for the DL section of the frame and carries information which allocates bursts within the DL part of the frame to individual stations. The UL-MAP carries information which allocates bursts within the UL part of the frame to individual stations and therefore defines when terminals can transmit.
There are restrictions on how the existing IEEE 802.16-2005 standard may be adapted to support relay stations. To ensure backwards compatibility with existing terminals, a relay station should appear to a terminal in the same manner as any other base station. This dictates that the relay station must also transmit a preamble set at the beginning of a downlink sub-frame, in the same manner as a base station. FIG. 3 shows a downlink sub-frame transmitted by a base station BS and a downlink sub-frame transmitted by a relay station RS, with both downlink sub-frames having a preamble set at the beginning of the frame. The transmissions of the base station BS and relay station RS must be synchronized. However, the relay station RS also needs to receive the preamble set 31 transmitted by the base station as this carries synchronisation information and information about which bursts within the frame are intended for the relay station. The scheme shown in FIG. 3 would require a relay station RS to receive preamble set 31 from the base station at the same time as transmitting a preamble set to terminals MS. This will increase the complexity of equipment at a relay station as it requires a relay station to have receiver equipment which can operate at the same time as the relay station is transmitting data. It also requires a high-level of isolation between the receive path and transmit path which may be difficult, or impossible, to achieve in many relay station installations.
One proposal made under the Wireless World Initiative New Radio (WINNER) project is to separate relay station and base station transmissions in the frequency domain, with each transmission using a separate block of OFDM sub-carriers. However, this can be wasteful of resources and also requires a high-level of isolation between transmit and receive paths which may be difficult to achieve.